Aggregation processes of fluorescent labeled molecules can be conveniently studied by FCS. The correlation amplitude is sensitive to the number of molecules contemporarily present in the measurement volume element, i.e. the focal spot of a microscope objective. Moreover, the diffusion characteristics are subject to change during aggregation thereby affecting the molecular residence time in the illuminated area. We investigated the time course of aggregation processes involving EGFP labeled membrane proteins linked to external receptors and stimulated by adding antigen on genetically engineered RBL cells.